A printed circuit board (PCB) is a composite of organic and inorganic materials with external and internal wiring that allows electronic components to be mechanically supported and electrically connected. A PCB may contain multiple insulation layers sandwiched between metal layers. A typical PCB contains at least one metal layer that is patterned to transfer signals between the electronic components, one power plane for distributing voltage throughout the PCB, and one ground plane.
Shielded transmission lines may be used to ensure that high-speed signals and radio frequency (RF) signals are transmitted without interference or corruption by other signals, and to ensure that these signals do not emanate unwanted electronic emissions. One way of insulating or protecting these signals is to use shielded cable that includes a central conductor within a plastic dielectric material surrounded by a braided conductor. Although this construction is extremely effective in transmitting signals over a distance, it may require cumbersome interconnects, and may not be useful for transmitting signals over small distances such as on a printed circuit board, for example. Numerous methods have been employed to create a shielded signal path on printed circuit boards. One option is to sandwich the signal conductor in the interior of the PCB between two outer ground planes. Unfortunately, this construction may only protect the signal conductor on the top and bottom sides but not the two lateral sides. In addition, this option may require considerable real estate on the PCB surfaces and significantly increase the laminate thickness.
Optical waveguides are another way to transmit signals. A waveguide transmits light or electromagnetic energy through an empty cavity or through a filled channel (as in glass or plastic fiber of a fiber optic cable). The waveguide may be coated with a highly reflective coating that causes the signal to be totally reflected from one side of the guide to the other. Waveguides may be used for communicating signals at all frequencies of the electromagnetic spectrum such as at radio frequencies and radar frequencies in addition to optical frequencies. It is desirable to form a waveguide within a printed circuit board so as to avoid problems caused by other transmission means and to enjoy the benefits of optical transmission.